Carbonyl reductase is a kind of oxidoreductase, and plays an important role in many biotransformation processes of biological organisms. Based on its capability in catalytically generating chiral alcohols with high enantioselectivity, the carbonyl reductase is usually applied as a very important biocatalyst to the synthesis of chiral intermediates in the chemical and pharmaceutical industries. DKR may stereoselectively reduce two carbonyls of a diketo acid ester simultaneously to give the corresponding hydroxyl, and it may be used for synthesizing key drug intermediates, particularly synthesizing a chiral dihydroxy hexanoic acid chain of a statin drug such as worldwide saleable cholesterol lowering drugs atorvastatin and rosuvastatin.
Currently known DKR can be used as a biocatalyst for reducing a diketone substrate in one step to prepare a chiral intermediate 3R,5S-dihydroxy-6-benzyloxy-tert-butyl hexanoate of a statin lipid-lowering drug with approximately single optical purity, thereby simplifying synthetic steps and reducing production pollution. However, application to industrial production still has some problems to be further solved. For example, low enzyme catalytic activity equates to a large amount of enzyme liquid and increased total volume of a reaction system, which increases production batches and production costs. These problems may be solved by directed evolution to improve catalytic activity of DKR. As a biocatalyst, an enzyme may fully develop its characteristics of high efficiency and high specificity in a biological system. However, there exist the common problems of inadaptability to an industrial production condition, low catalysis capability of an unnatural substrate and the like during industrial applications. Enzyme molecules are required to be modified to meet different application requirements by virtue of protein engineering methods. The protein engineering methods may be summarized into three: rational design, irrational design and semi-rational design.
Rational design refers to changing individual amino acids in protein molecules by virtue of site-directed mutagenesis or another methods on the basis of knowing the spatial structure of proteins, thereby generating proteins with new characters. This method is theoretically high in pertinence, and is mainly used for modifying catalytic activity of natural apoenzymes, substrate specificity and stability, changing an inhibitor type, coenzyme specificity and the like.
A site-directed saturation mutagenesis technology is an important technology in protein engineering, belongs to semi-rational design, but combines advantages of rational design and irrational design, overcomes respective shortcomings, and modifies a coding gene of a target protein to acquire a mutant of which an amino acid at a target site is substituted with other 19 amino acids respectively within a short time. This technology not only is a powerful tool for directed modification of proteins, but also is an important means for researching a protein's structure-function relationship. Researches show that multisite mutagenesis may always obtain an evolution more ideal than that obtained by single-site mutagenesis. Multisite mutagenesis is unlikely to be directly implemented by site-directed mutagenesis. However, site-directed saturation mutagenesis can increase the diversity of mutants and is simple to manipulate.
Therefore, if a site-directed saturation mutagenesis technology may be utilized to modify DKR to improve its catalytic activity and substrate specificity and/or stability, the problems of large amount of enzyme liquid, high production cost and the like in the prior art may be solved.